It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.
Gold is generally extracted from gold containing ores by treatment with cyanide solution, which solubilises the gold. However, in some ores, the gold is present as microparticles encapsulated within the ore. The gold in such ores cannot be extracted by traditional cyanidation techniques. These types of ores are known as refractory ores and are typically sulphide and/or carbonaceous ores. These ores may also contain, along with sulphides, other compounds of other Group VIA elements such as selenium and tellurium and the Group VA elements such as Sb and Bi.
In order to extract gold from refractory sulfide ores, the ores must first be treated to liberate the gold so as to be accessible to cyanide leaching. A refractory ore is typically treated by oxidizing the ore which results in the chemical destruction of the refractory component of the ore, liberating precious metals for subsequent recovery. Known methods of oxidising refractory ores include roasting, bacterially assisted leaching and leaching the ore at elevated temperatures and pressure under acidic conditions.
Leaching of nickel and cobalt under alkaline conditions using ammonia/ammonium salts is known. However, a major disadvantage of alkaline leaching is that when iron and arsenic containing ores such as pyrite and arsenopyrite are oxidised, the iron and arsenic which are leached precipitate as a passive oxide and/or sulphur rich layer on the mineral particle. This layer inhibits further oxidation with the result being that the extent of leaching under alkaline conditions is less than under acidic conditions. This translates to a lower recovery of precious metals.
Still further, alkaline leaching of refractory materials requires elevated pressure and temperatures and an oxidant for the leaching to occur. However, even under aggressive alkaline conditions, recovery of precious metals is often less than that for acid leaching. Further, base metals such as copper and zinc are insoluble at high pH. Thus, alkaline leaching is unsuitable for leaching ores or concentrates where recovery of base metals from base metal sulphides such as chalcocite, sphalerite or chalcopyrite is required. For these reasons, commercial and academic interest has been directed towards acid leaching.
Most of the literature relating to alkaline leaching is directed towards the use of water soluble alkalis such as sodium or potassium hydroxide and ammonia. A disadvantage with these reagents is that iron is precipitated primarily as jarosite. Jarosite inhibits gold recovery and is also an environmentally unacceptable residue. Also, hydroxide reagents and in particular sodium hydroxide are prohibitively expensive.
The use of cheaper alkalis such as lime has been proposed. However, to date, leaching of iron sulphide materials with lime has been unsuccessful in that leaching is incomplete and subsequent precious metal recovery is low. For example, an earlier study of alkaline oxidation of pyrite for gold recovery using lime achieved only 30 to 40% gold recovery which offered little improvement over direct cyanidation of the pyrite. This is believed to be due to passivation of the mineral by precipitation of a gypsum/iron oxide layer.
Limestone is another alkali which is relatively cheap. Limestone is typically used in the neutralization of acidic leachates. However, limestone is considered to be insufficiently reactive and/or soluble in alkaline systems to be able to be used for alkaline leaching.
As mentioned above, it is known that the oxidation rate under acidic conditions can be increased by fine grinding to increase the surface area of the mineral particles. Such an increase may be predicted given that there is a larger surface area exposed to the oxidizing agents. However in the alkaline system, this effect is substantially reduced in view of the formation of the passive oxide and/or sulphur rich layer on the particles. The rate determining factors in the alkaline systems are believed to relate to the formation of the passivating oxide and/or sulphur rich layer and diffusion of reactants through the layer. Thus, workers in the field have concentrated on increasing the extent of alkaline leaching by using strong, soluble alkalis, by modifying the leaching conditions so as to minimise formation of the passive layer and/or influence the diffusion rate through the layer.
One study suggests leaching at higher temperatures or at relatively concentrated solutions of reagents. The reason for this is to rapidly produce a passive layer which is unstable and subject to cracking. It is believed that at lower temperatures, the layers grow more slowly and are more stable. Another suggestion has been to use additives which may react to dissolve the layer or to make the layer more permeable.
In Australian patent number 744356 (which corresponds to U.S. Pat. No. 6,833,021), the entire contents of which are herein incorporated by cross-reference, a method of processing a mineral composition comprising a refractory material is disclosed. The method comprises milling the composition to a particle size of P80 of less than 25 μm and leaching the composition with a solution comprising lime and/or limestone in the presence of an oxygen containing gas. The specific conditions disclosed in this patent for processing the mineral composition include conducting the leaching step at a pH of from 6 to 12, or preferably from 6 to 9. The examples given in this patent utilise a pH of 8, 9 or 10 in the leaching step. The alkaline material added to the leaching step in the examples of this patent comprises lime or a mixture of lime and limestone. The method is described as being useful for recovering precious metals from a mineral composition comprising a refractory material. The method of this patent is described as being useful for treating mineral compositions that include pyrite or arsenopyrite.